The present invention relates to an improvement in an apparatus for observing a scanning electron microscope image and a transmission optical microscope image of the same specimen.
When observing cells of biological specimens, comparisons may be made between an image obtained by an optical microscope (hereinafter an "OM" and an image obtained by a scanning electron microscope (hereinafter an "SEM" ). The fields of view of both images are made coincident with each other. In this method of specimen observation, a plate of glass is coated with a thin transparent electrically conductive film and a thin specimen is mounted thereon. The specimen and plate are placed in a chamber kept at a vacuum. The OM image is obtained by illuminating the specimen with visible light. The SEM image is obtained by irradiating the specimen with an electron beam to produce backscattered electrons or secondary electrons which are detected to generate an image signal which is used to modulate the electron beam in a cathode-ray tube. It is a disadvantage, from the standpoint of maintaining a desired resolution, to increase the distance between the objective lens of the OM and the specimen. In most applications, therefore, it is customary to separate the objective lens of the OM from the eyepiece thereof, to place the objective lens in the specimen chamber close to the specimen and to place the eyepiece outside the specimen chamber. With this arrangement, however, it is quite difficult to achieve axial alignment between the eyepiece and the objective lens. Further, when a larger-size specimen is observed by the SEM, the OM and a specimen moving mechanism for OM observations must be removed from the specimen chamber for replacement with another specimen moving mechanism which is suited for observing specimens of larger sizes. This procedure is complex and cumbersome.